The Moving Picture Experts Group phase 2 (MPEG-2) standard is a digital audio/video (A/V) compression standard employed in a variety of audio/video distribution systems including, for example, Digital Satellite System (DSS) broadcasting. The MPEG-2 transport standard, ISO 13818-1, requires the broadcaster to transmit a program clock reference (PCR) time stamp within the multiplexed audio and video packet stream at periodic intervals. This program clock reference time stamp, referred to as a system clock reference (SCR) in the DSS program stream, bears a strict relationship to the system time clock(s) (STC) within the MPEG-2 encoder generating the broadcast stream, and therefore may be employed to replicate the encoder's system time clock in the decoding equipment. Additionally, each audio and video packet multiplexed into the MPEG-2 broadcast stream contains a decoding time stamp (DTS) and a presentation time stamp (PTS), which identify the times, relative to the program clock reference, at which the packet must be decoded and presented by the decoding equipment for display, respectively.
Implementation of personal video recording (PVR) involves storing the broadcast programs on storage media and then playing them back. If the programs are stored in their entirety, as MPEG-2 streams and played back by injection at the original broadcasters bit-rate, the timing information is retained allowing the delayed program to use the same audio/video synchronization mechanism as the live broadcast. Storage of MPEG-2 streams in their entirety requires prohibitively large amounts of storage, However, if the MPEG-2 streams are altered or compressed by discarding programs or if programs are altered or compressed by discarding sub-channels, storage requirements are reduced but the critical timing of the arrival of the PCRs is lost. Storage of only audio elementary streams (AES) and video elementary streams (VES) also reduces storage requirements but again results in loss of the PCR arrival timing. Loss of the PCR arrival timing results in serious audio/video synchronization problems evident in such phenomenon as noise, breaks or pauses in the audio/video presentation during playback.